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Animals that travel together in groups display a variety of
fascinating motion patterns thought to be the result of delicate local
interactions among group members. Although the most informative way of
investigating and interpreting collective movement phenomena would be afforded
by the collection of high-resolution spatiotemporal data from moving
individuals, such data are scarce and are virtually non-existent for
long-distance group motion within a natural setting because of the associated
technological difficulties. Here we present results of experiments
in which track logs of homing pigeons flying in flocks of up to 10 individuals
have been obtained by high-resolution lightweight GPS devices and analysed using
a variety of correlation functions inspired by approaches common in statistical
physics. We find a well-defined hierarchy among flock members from data concerning
leading roles in pairwise interactions, defined on the basis of characteristic
delay times between birds' directional choices. The average spatial position of
a pigeon within the flock strongly correlates with its place in the hierarchy,
and birds respond more quickly to conspecifics perceived primarily through the
left eye – both results revealing differential roles for birds that assume
different positions with respect to flock-mates. From an evolutionary perspective,
our results suggest that hierarchical organization of group flight may be more
efficient than an egalitarian one, at least for those flock sizes that permit
regular pairwise interactions among group members, during which leader-follower
relationships are consistently manifested.